Magical Explorer, Vol. 1 (manga): Reborn as a Side Character in a Fantasy Dating Sim (Volume 1) (Magical Explorer (manga), 1)

The legendary game full of pretty girls: Magical Explorer!
Getting transported into your favorite harem romance game is the dream, right? But when one poor guy is cast as the main character's BEST FRIEND, it's more of a living nightmare! After all, living as Kousuke Takioto means watching his best friend win over all the girls--and getting none for himself. That's how the script goes, anyway. But if he uses his full knowledge of the game to his advantage, maybe there's still a chance! A chance to turn his nightmare into his own private paradise...